The third round of talks between Iran and five UN Security Council permanent members - Britain, China, France, Russia, the United States - plus Germany, is a bid to draft a comprehensive agreement that will allow Iran to continue its nuclear development program while reassuring the powers it will not be used for military purposes.

"They are doing detailed work on issues," said Michael Mann, spokesperson of EU foreign policy chief Catherine Ashton, noting the atmosphere of the meeting was good so far.

Both U.S. and Iranian officials have voiced the hope of starting the drafting of an agreement in May.

The detailed discussions are based on a six-month interim deal reached in Geneva last December.

Iranian Foreign Minister Mohammad Javad Zarif, the chief Iranian negotiator in the talk, said the six months' time is adequate for discussion to make a comprehensive agreement on the Iranian nuclear program, if sides involved come with "good faith and will," after his meeting with Ashton Monday evening in Vienna.

The Crimea issue, a wedge between Russia and Western countries, is seen as an element which could affect the diplomatic process in Vienna.

Russia's chief negotiator Sergei Ryabkov warned last month that Moscow might alter its position on the Iran talks if pushed too far.

Russia and Iran were reported to be negotiating an oil-for-goods trade that would weaken the West's sanctions policy, a strategy pressing Iran to make compromise on the negotiation table.

The talks so far have been difficult in dealing with how large the scale could Iran preserve its nuclear program, ensuring its peaceful nature while respecting Tehran's nuclear right under the NPT.

A lot of Iran's nuclear infrastructure is dual-use facilities, including the Arak IR-40 heavy water reactor and Fordo nuclear plant, remaining key concerns of the Western countries.

The West has long suspected Iran is developing nuclear weapons under the cover of a civilian atomic plan, while Tehran says its nuclear program is solely peaceful.

Tehran has announced its no-abandon nuclear facilities policy, saying its nuclear right should be respected.

Western countries fear that Iranian domestic hardliners could press Hassan Rouhani's government to leave the negotiation table if diplomacy poses too much pressure on Iran to scale back its nuclear plan significantly.

But leaving too much of Iran's nuclear infrastructure intact would also be hard to win the support of Israel and conservative U.S. lawmakers